[AIO] kiocb locking to serialise retry and cancel

Implement a per-kiocb lock to serialise retry operations and cancel.  This
is done using wait_on_bit_lock() on the KIF_LOCKED bit of kiocb->ki_flags.
Also, make the cancellation path lock the kiocb and subsequently release
all references to it if the cancel was successful.

Signed-off-by: Benjamin LaHaise <benjamin.c.lahaise@intel.com>
diff -purN --exclude=description 30_aio_write/fs/aio.c 40_lock_kiocb/fs/aio.c
--- 30_aio_write/fs/aio.c	2005-06-20 13:33:31.000000000 -0400
+++ 40_lock_kiocb/fs/aio.c	2005-06-20 17:16:08.000000000 -0400
@@ -545,6 +545,24 @@ struct kioctx *lookup_ioctx(unsigned lon
 	return ioctx;
 }
 
+static int lock_kiocb_action(void *param)
+{
+	schedule();
+	return 0;
+}
+
+static inline void lock_kiocb(struct kiocb *iocb)
+{
+	wait_on_bit_lock(&iocb->ki_flags, KIF_LOCKED, lock_kiocb_action,
+			 TASK_UNINTERRUPTIBLE);
+}
+
+static inline void unlock_kiocb(struct kiocb *iocb)
+{
+	kiocbClearLocked(iocb);
+	wake_up_bit(&iocb->ki_flags, KIF_LOCKED);
+}
+
 /*
  * use_mm
  *	Makes the calling kernel thread take on the specified
@@ -773,7 +791,9 @@ static int __aio_run_iocbs(struct kioctx
 		 * Hold an extra reference while retrying i/o.
 		 */
 		iocb->ki_users++;       /* grab extra reference */
+		lock_kiocb(iocb);
 		aio_run_iocb(iocb);
+		unlock_kiocb(iocb);
 		if (__aio_put_req(ctx, iocb))  /* drop extra ref */
 			put_ioctx(ctx);
  	}
@@ -1523,6 +1543,7 @@ int fastcall io_submit_one(struct kioctx
 			;
 	}
 	spin_unlock_irq(&ctx->ctx_lock);
+	unlock_kiocb(req);
 	aio_put_req(req);	/* drop extra ref to req */
 	return 0;
 
@@ -1648,6 +1669,7 @@ asmlinkage long sys_io_cancel(aio_contex
 	if (NULL != cancel) {
 		struct io_event tmp;
 		pr_debug("calling cancel\n");
+		lock_kiocb(kiocb);
 		memset(&tmp, 0, sizeof(tmp));
 		tmp.obj = (u64)(unsigned long)kiocb->ki_obj.user;
 		tmp.data = kiocb->ki_user_data;
@@ -1659,6 +1681,13 @@ asmlinkage long sys_io_cancel(aio_contex
 			if (copy_to_user(result, &tmp, sizeof(tmp)))
 				ret = -EFAULT;
 		}
+		unlock_kiocb(kiocb);
+		/* If the cancellation was successful, we must discard the
+		 * reference held for completion of the iocb.
+		 */
+		if (!ret)
+			aio_put_req(kiocb);
+		aio_put_req(kiocb);
 	} else
 		printk(KERN_DEBUG "iocb has no cancel operation\n");